Recently, electrolytes have a high tendency to be used in the form of solid rather than the conventional form of (aqueous) solution. This is because firstly those solid electrolytes have good processability so that they can be easily applied in electric and electronic components, and secondly there are trends for reduction of weight, thickness, length and size of such components and further for power saving.
Proton conductive materials, both inorganic and organic, are known in the art. However, inorganic proton conductive materials, such as uranyl phosphate hydrates, come with many difficulties when superposed as a conductive layer onto a substrate or an electrode. For example, sufficient contact cannot be achieved in the interface between the conductive layer and the substrate, etc.
On the other hand, organic proton conductive compounds may be exemplified with polymers that belong to the cation exchange resins, e.g. sulfonated vinyl polymers such as polystyrene sulfonic acid; perfluoroalkylsulfonic acid polymers, which are typically represented by Nafion® (Du Pont Kabushiki kaisha); perfluoroalkylcarboxylic acid polymers; and heat resistant polymers, including polybenzimidazole and polyether ether ketone, in which sulfonic or phosphoric groups have been introduced (Polymer Preprints, Japan, Vol. 42, No. 7, p. 2490–2492 (1993), Polymer Preprints, Japan, Vol. 43, No. 3, p. 735–736 (1994), Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)).
Sulfonated polyarylenes also are known as proton conductive materials; they are generally obtained by polymerizing an aromatic compound and reacting the resultant polymer with a sulfonating agent to thereby introduce a sulfonic group into the polymer.
In the current sulfonation methods, the sulfonating agents, including concentrated sulfuric acid, fuming sulfuric acid and chlorosulfuric acid, have to be used in large amounts. This increases production risks and limits the variety of available plant materials. Further, cumbersome work is required to carry out a wastewater treatment after the polymer has been recovered. Also, it is difficult with the conventional sulfonation methods to control the amount and position of sulfonic groups introduced into the polymer.